Henry McLeish said Mr Salmond “cannot have his political cake and eat it” by asking voters to support both independence and so-called ‘devolution max’, whereby all tax powers would be transferred to Holyrood.

Instead the former Labour First Minister said the two propositions would go head to head, with the winner being the option that receives the most votes.

He predicted ‘devo max’ would be the most popular choice and Mr Salmond would have a much better chance of winning full independence if the second question was left off the ballot paper.

The intervention is a blow to the SNP leader, who has dropped heavy hints he wants people to vote ‘yes, yes’ in the referendum for both extra powers and independence. This would give him a ‘consolation prize’ if voters reject full separation.

Controversially, Mr Salmond has claimed that Scotland would leave the United Kingdom if independence emerges as voters’ second choice but attracts more than 50 per cent support.

This would apply even if ‘devo max’ won 99 per cent backing. However, Professor John Curtice, Britain’s leading elections expert, yesterday rejected this analysis and agreed the victor would be the most popular option.

Mr McLeish, who succeeded Donald Dewar as First Minister, said: “Alex Salmond is a sharp operator but cannot have his political cake and eat it. You cannot be a pro-independence party and a pro-devo max party.

“It’s one against the other. I think independence cannot win and that’s fine by me. Voters cannot tick both boxes because that would present a win, win situation for the Scottish government – that’s a nonsense.”

The First Minister this week announced his referendum plans and confirmed he was open to a second question appearing on the ballot paper.

A campaign involving groups including the Church of Scotland, the STUC and the Scottish Council for Voluntary Organisations is being announced on Monday to examine a possible ‘extra powers’ question.

Mr McLeish dismissed reports he would lead it but said he would support moves to present voters with a “Home Rule” choice other than “the same old tired Unionism and the same old tired independence”.

He also rejected Mr Salmond’s comparison with the 1997 devolution referendum, where voters were encouraged to vote ‘yes, yes’ to two questions.

The first was on creating a Scottish Parliament and the other on giving it tax-varying powers. However, the SNP’s referendum plans would involve two alternative proposals of equal weight.

Prof Curtice, professor of politics at Strathclyde University, said: “Devolution max and independence are two separate categories so we would have to go with whichever one was the most popular, even if there was more than 50 per cent support for both.”

He said its inclusion on the ballot paper would “significantly reduce the chances of independence winning”.

For this reason, many of the SNP’s rank-and-file members agree with David Cameron that there should be single, straight question on independence.